Subducted carbon weakens the forearc mantle wedge in a warm subduction zone

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

(Supplementary Fig. 1d).Our prediction shows carbonates (calcite or dolomite; 5.6 vol%) are dominant as compared with graphite (0.5 vol%) from 0.6 to 2.0 GPa, whereas only graphite (0.5 vol%) is present from 2.2 to 2.5 GPa.e predicted rock carbon contents remain largely unchanged from 1.1 wt% at 0.4 GPa to 0.9 wt% at 2.0 GPa (Supplementary Fig. 1d), and then decrease to 0.4 wt% at 2.0-2.2GPa, indicative of the dissolution of significant amounts of carbon in the fluids.e predicted ΔFMQ values of the fluids decrease gradually from +0.5 (at 20 km) to -0.1 (at 73 km), and decrease abruptly to -0.9 (at 80 km depth; Supplementary Fig. 1e).X H2O varies from 0.88 to 1.00 with increasing depth from 25 to 90 km.X CO2 increases with depth from 0.002 at 20 km to 0.1 at 70 km (Supplementary Fig. 1f).

Influence of t/L on model predictions
Our model prediction was affected by the parameter t/L (Myr m -1 ) in equation ( 4), where t (Myr) is the timescale of fluid infiltration and L (m) is the length or thickness of the forearc mantle basement normal to the slab dip.Here we compare the model predictions for t/L = 0.1, 1, and 10 (Myr m -1 ).We considered two types of model scenarios.e first type simulates an increasing amount of fluid entering the reacting serpentinite (herein, the "batch" calculation), which was used to obtain the results presented in the main manuscript (Fig. 2e-h).e second type simulates the fractionation of fluid at each step of fluid addition (herein, the "fractional" calculation), simulating the evolution of the bulk composition of the rock in a relatively open system.e batch and fractional calculations were performed by mode 0 and mode 1 calculations in the VERTEX subprogram in Perple_X, respectively.e calculations were performed along the slab-top geotherm of the northeastern Japan and Nankai subduction zones 1 .e reacting fluids were those fluids in equilibrium with the subducting sediments in the northeastern Japan (Fig. 2a) and Nankai (Fig. 2b) subduction zones.
In the northeastern Japan subduction zone, the results for the batch (Supplementary Fig. 2a, b) and fractional (Supplementary Fig. 2c, d) calculations in the case of t/L = 0.1 Myr m -1 show no significant difference from the reactant (i.e., serpentinite).At 74 km depth, the batch calculation (Supplementary Fig. 2a, b) with t/L = 0.1 Myr m -1 did not yield a result due to numerical instability, but the fractional calculation with t/L = 0.1 Myr m -1 yields a talc + magnesite assemblage (Supplementary Fig. 2c, d).e batch calculation with t/L = 1 Myr m -1 (Supplementary Fig. 2e and f) yields an identical to that in Fig. 2e and f, respectively, and the results are described in the main manuscript.e results are largely similar to the result of the fractional calculation with t/L = 1 Myr m -1 (Supplementary Fig. 2g and h).e batch calculation with t/L = 10 Myr m -1 (Supplementary Fig. 2i, j) did not yield a result due to numerical instability.e fractional calculation with t/L = 10 Myr m -1 (Supplementary Fig. 2k, i) yields a talc + quartz assemblage at 68-80 km depth, with minor carbonate (Supplementary Fig. 2i).
For the Nankai subduction zone, the results of the batch calculation with t/L = 0.1 Myr m -1 show no significant difference from the reactant (i.e., serpentinite).A small amount of chlorite and amphibole occur at 55-60 km depth (Supplementary Fig. 3a).A small amount of carbonate (magnesite) was also predicted at 35-78 km depth (Supplementary Fig. 3b).e results of the fractional calculation with t/L = 0.1 Myr m -1 show similar trends to the batch calculation (Supplementary Fig. 3c, d).e batch calculation with t/L = 1 Myr m -1 (Supplementary Fig. 3e, f) yields the same results as in Fig. 2g, h, respectively.e results are described in the main manuscript.e fractional calculation with t/L = 1 Myr m -1 (Supplementary Fig. 3g, h) yields similar results to those of the batch calculation.e talc fraction is relatively low around the MWC and increases with depth.e proportions of talc and magnesite increase in a similar manner with increasing depth (Supplementary Fig. 3h).e batch calculation with t/L = 10 Myr m -1 (Supplementary Fig. 3i, j) did not yield a result due to numerical instability.e fractional calculation with t/L = 10 Myr m -1 shows that the talc fraction decreases and the quartz fraction increases with increasing depth (Supplementary Fig. 3k, l).e occurrence of talc + carbonate rock is predicted at 35-47 km depth, and carbonate (magnesite) + quartz rock at 54-74 km depth (Supplementary Fig. 3k, l).
Taking t/L = 10 Myr m -1 , the batch calculations for the northeastern Japan (Supplementary Fig. 2i, j) and Nankai (Supplementary Fig. 3i, j) subduction zones did not yield a result, whereas the fractional calculations yielded results (Supplementary Figs.2k, l and 3k, l).Overall, the batch and fractional calculations yield similar results for both subduction zones.erefore, although the results provided in the main manuscript (Fig. 2e-h) were obtained from the batch calculations, we can use the result of fractional calculations for various t/L values (0.1, 1, and 10) to discuss the mineral assemblage at a range of t and L values.

Model results regarding the infiltration of sediment-derived fluid into anhydrous forearc mantle
e model results regarding the interaction of fluids derived from subducted sediment and hydrous mantle wedge (serpentinite) are presented in the main manuscript.Similar calculations were conducted for an anhydrous mantle wedge.e mantle wedge peridotite was modeled using the composition of depleted mid-ocean-ridge-basalt sourced mantle (DMM) 2 (Supplementary Table 1).e Fe 3+ /∑Fe ratio was set to 0.035 3 and t/L was set to 1 (Myr m -1 ).
e results for the northeastern Japan and Nankai subduction zones are largely similar to the results obtained when using a serpentinite composition (Fig. 2e-h).For northeastern Japan, talc + chlorite + magnesite is predicted at 70-72 km depth and chlorite at 75-90 km depth (Supplementary Fig. 4a, b).In the Nankai subduction zone, talc is not predicted to occur at 35-40 km depth around the MWC, but the proportion of talc increases from 5 vol% at 41 km depth to 59 vol% at 73 km (Supplementary Fig. 4c).

Modeling the dehydration of subducting altered oceanic crust
Fluids released from subducting altered oceanic crust (AOC) would infiltrate the mantle wedge, along with the fluids from subducting sediment [4][5][6] .Inorganic carbon in AOC occurs mainly in the upper basaltic portion of the crust [7][8][9][10] (300 m thickness) 11,12 .We modelled the stable mineral assemblages along the slab-top geotherm of the northeast Japan and Nankai subduction zones using a typical carbonated AOC basalt 13 (Supplementary Table 1).We used a H 2 O-CO 2 solvent for modeling AOC in both subduction zones.Solute CO 2 species (CO 2,aq ) were excluded from the calculations.e solid-solution models used for the AOC calculation are listed in Supplementary Table 3.
To calculate the fluid flux (J; kg fluid m -2 Myr -1 ) derived from dehydration of the AOC during subduction, a similar equation was used as that for calculating the fluid flux from sediment (equation 3 in the main text), rewritten for AOC as follows: where ρ AOC (kg m -3 ) is the density of the AOC, h (m) is the thickness of the AOC, v (m Myr -1 ) is the subduction velocity, and l (m) is the along-dip length of the slab.Δm fluid is the change in fluid mass concentration (kg fluid kg -1 rock) over interval l. e v values were set to 100,000 and 40,000 m Myr -1 for northeastern Japan and Nankai, respectively 14 .e l value was calculated from the depth and horizontal distance of the thermal structures in northeastern Japan and Nankai.We assumed that h and v are constant.
In northeast Japan, the H 2 O content (4.5 wt%) shows a minimal decrease from 25 to 70 km depth (Supplementary Fig. 5a), but a decrease from 4.5 wt% at 70 km to 0.4 wt% at 80 km depth due to the breakdown of lawsonite + pumpellyite.In contrast, in the Nankai subduction zone, decreases in H 2 O content are predicted at depths of ~30 and 58-90 km due to the breakdown of chlorite and chlorite + epidote, respectively (Supplementary Fig. 5b).
e total flux was used to constrain the mineral assemblage in the northeastern Japan and Nankai subduction zones, with t/L set to 1 (Myr m -1 ).In northeastern Japan, serpentine and chlorite are predicted to occur at depths of 68 and 77-90 km, respectively (Supplementary Fig. 5e), without carbonate minerals (Supplementary Fig. 5f).Compared with the model results that consider only sediment dehydration (Fig. 2e, f), the total flux shows greater amounts of talc and chlorite at depths of >70 km.In the Nankai subduction zone, the model calculation suggests that the proportion of talc increases with depth (Supplementary Fig. 5g), as also observed for the model results considering the dehydration of sediment (Fig. 2g, f).e proportion of talc increases from 0.6-5.0vol% around the MWC (35-40 km depth) to 56 vol% at 60 km depth (Supplementary Fig. 5g).
Compared with the results obtained when considering only sediment dehydration (Fig. 2g), an increase in the proportion of talc is predicted at depths of 58-63 km (Supplementary Fig. 5g) as a result of fluid flux from AOC (Supplementary Fig. 5d, g).At 68 km depth, where the total flux shows a peak, magnesite + quartz occur without hydrous minerals (Supplementary Fig. 5h).e results indicate magnesite occurrence without hydrous minerals at depths of 82-90 km.
In the case that fluid fluxes from AOC are considered, enhanced fluid fluxes are predicted at depths of >72 km and >58 km in the northeastern Japan (Supplementary Fig. 5c) and Nankai (Supplementary Fig. 5d) subduction zones.e predicted depth of AOC dehydration is largely consistent with previous studies 6 .Because dehydration of AOC does not occur around the MWC in the Nankai subduction zone, we conclude that the coupled dehydration of subducting sediment and AOC may enhance the contrast between a talcpoor region around the MWC and a talc-rich region at greater depths.

Effect of underplating of subducting sediments
In the model, we assumed that the thickness of subducted sediment (h) remains similar with depth, although sediment underplating may reduce the flux of sediment delivery to the deep Earth.e thickness of subducting sediment at forearc to subarc depths is geophysically unobservable, but on-land exposures of paleo-subduction zones suggest that underplated sediment makes up 20%-85% of the total subducted sediment [15][16][17] .With increasing proportion of underplated sediment, the fluid flux into the overlying forearc mantle would originate mainly from AOC.In the Nankai subduction zone, the calculation with AOC shows that the fluid flux around the MWC (35 km) and at greater depths (>58 km) is mainly from subducting sediment and AOC, respectively (Supplementary Fig. 5d).erefore, with increasing proportion of underplated sediment, the fluid flux around the MWC may decrease, while that at greater depths may show little change.ese depth variations in fluid flux related to sediment underplating would enhance the heterogeneity in talc distribution.Consequently, sediment underplating also results in a heterogeneous distribution of talc, and subduction megathrusts in warm subduction zones are expected to show down-dip variations in talc occurrence.

Calculations with and without carbon
Based on calculations of the infiltration of sediment-derived fluid into the mantle rocks under various P-T conditions along the slab top in Nankai and northeastern Japan, we estimated the amount of fluid required for the appearance of talc (ξ Tlc ) (Supplementary Fig. 6).In the northeastern Japan subduction zone, ξ Tlc first increases from 360 mol kg -1 at 0.6 GPa to 1000 mol kg -1 at 0.7 GPa, and then decreases to 106 mol kg -1 at 2.0 GPa in the carbon-bearing system.Even if carbon is excluded from the system, ξ Tlc does not change significantly, ranging from 30 to 400 mol kg -1 at pressures of 0.6 to 2.5 GPa.In the Nankai subduction zone, ξ Tlc decreases with increasing P and T, from ξ = 166 mol kg -1 at 0.6 GPa to 9 mol kg -1 at 2.1 GPa (Supplementary Fig. 6).For comparison, similar calculations were conducted with carbon-free sediments, resulting in ξ Tlc = 327 mol kg -1 at 0.6 GPa to ξ Tlc = 41 mol kg -1 at 2.1 GPa (Supplementary Fig. 6).e obtained ξ Tlc values are two to four times higher than in the carbon-bearing case.

Parameter studies on subducted carbon in sediment
e key variables in the present study are the subduction geotherm (i.e., P-T conditions), whole-rock sediment composition, total carbon (TC) contents, and fraction of organic carbon relative to TC (F OC ).ese parameters vary in each subduction zone.We conducted thermodynamic calculations using various parameters to understand the sensitivities of the parameters to metasomatism of the mantle wedge.e calculation settings are summarized in Supplementary Table 4. e calculations used the global average of subducting sediments (GLOSS) 18 as the whole-rock composition of the sediments.GLOSS contains 3.01 wt% CO 2 , which corresponds to ~8200 mg kg -1 TC.
Because the F OC of GLOSS is unknown, we conducted thermodynamic calculations using GLOSS for selected F OC values (0.1, 0.5, and 0.9) at the geotherm of Nankai and northeastern Japan (calculations 1-6 in Supplementary Table 4).Moreover, we conducted thermodynamic calculations using GLOSS with modified TC values: GLOSS with 4100 mg kg -1 TC (calculations 7-12 in Supplementary Table 4) and 16,200 mg kg -1 TC (calculations 13-18 in Supplementary Table 4) for selected F OC values (0.1, 0.5, and 0.9).
ese calculations constrain the effects of the subduction geotherm, TC, and F OC .e bulk rock composition used for each calculations were listed in the Supplementary Table 5.
Concentrations of carbon in the fluids (m C ) equilibrated with the sediments are shown in Supplementary Fig. 7a.Despite the changes in TC and F OC , the concentrations of carbon are broadly similar for each subduction zone.For example, in the Nankai subduction zone, the m C values for TC = 4100, 8200, and 16400 ppm, and 0.6-2.2GPa are 0.5-4.1,0.5-6.1, and 0.5-7.4mol kg -1 , respectively.In the northeastern Japan subduction zone, the m C values for TC = 4100, 8200, and 16400 ppm, and 0.6-2.2GPa are 0.1-0.5, 0.1-0.6, and 0.2-0.9mol kg -1 , respectively.In both subduction zones, F OC = 0.1 leads to relatively high m C values as compared with F OC = 0.5 and 0.9 (Supplementary Fig. 7a-c).
In all calculations, m C is high in the Nankai subduction zone (warm subduction zone) as compared with the northeastern Japan subduction zone (cold subduction zone), suggesting the subduction geotherm controls m C in metamorphic fluids.
Concentrations of Si in the fluids (m Si ) equilibrated with the sediments are shown in Fig. S7d-f.In the Nankai subduction zone, less variation of m Si values with TC and F OC are observed, ranging from 0.07 to 1.0 mol kg -1 at 0.6-2.2GPa, F OC = 0.1-0.9, and TC = 4100-16,400 ppm (Supplementary Fig. 7d-f).In the northeastern Japan subduction zone, m Si values vary from 0.03 to 0.15 mol kg -1 at 0.6-2.2GPa, F OC = 0.1-0.9, and TC = 4100-16,400 ppm.At 8200 and 16,400 ppm TC, m Si values at F OC = 0.1 are lower than those at F OC = 0.5 and 0.9 (Supplementary Fig. 7e-f Even at the same geotherm, a low F OC value likely decreases the ξ Tlc value.For example, in the Nankai subduction zone, or for a calculation with sediments with 0.41 wt% TC (Supplementary Fig. 7g), the ξ Tlc values for F OC = 0.5 and 0.9 are similar, but the ξ Tlc values for F OC = 0.1 are lower, especially at lower P (<1.0 GPa).is trend is also observed when TC is increased (Supplementary Fig. 7h-i) in the northeastern Japan subduction zone (Supplementary Fig. 7g-i).erefore, low F OC (i.e., carbonate-dominated sediments) may enhance talc formation in the mantle wedge.

Effect of sediment type on talc formation in the forearc mantle
We conducted parameter studies to investigate the effect of sediment type on talc formation in the forearc mantle.We considered five types of sediment: carbonate sediment, chert, pelagic clay, terrigenous sediment, and turbidite.
e chemical compositions of the sediments are listed in Supplementary Table 6.e bulk rock composition of the carbonate sediment was taken from site 495 in Guatemala, from Plank and Langmuir 18 .We assumed that all carbon in the carbonate sediment was present as carbonate minerals (F OC = 0.00).For the other sediment types (chert, pelagic clay, terrigenous sediment, and turbidite), we combined the concentrations of organic and inorganic carbon reported by Cli 14 with the bulk compositions of Plank 19 .For chert and pelagic clay, we used the bulk rock compositions from site 801 at the Mariana Islands 18 , along with organic and inorganic carbon concentrations from this site 14 .For terrigenous sediment, we used the bulk rock composition from the Antilles 18 , and organic and inorganic carbon concentrations from the "Lesser Antilles mean" from Cli 14 .For turbidite, we used the bulk rock composition from site 178 in Alaska 18 along with organic and inorganic carbon concentrations from the same site 14 .e Fe 3+ /∑Fe ratio was set to 0.23, based on the global average for metapelites 20 .e calculations were conducted along the slab-top geotherm of the northeastern Japan and Nankai subduction zones 1 .
For northeastern Japan, the calculated fluids in equilibrium with the five types of sediment yield higher values of m C (0.1-2.0 mol kg -1 ) than m Si (0.004-0.1 mol kg -1 ; Supplementary Fig. 8a).e chert has the highest m C values (~2.0 mol kg -1 ), whereas the carbonate sediment has the lowest (0.1-0.2 mol kg -1 ).For the Nankai subduction zone, m C values (0.5-4.8 mol kg -1 ) are an order of magnitude higher than m Si values (0.06-0.6 mol kg -1 ; Supplementary Fig. 8b).e chert has the highest m C values (2.0-4.8 mol kg -1 ), followed by pelagic clay (0.9-4.8 mol kg -1 ).e m Si values are similar for the five types of sediment (Supplementary Fig. 8b).
Using the calculated fluid compositions, we calculated the infiltration of sedimentderived fluid into the mantle rocks beneath the slab-top at the northeastern Japan and Nankai subduction zones.For northeastern Japan, the calculated ξ Tlc values (Supplementary Fig. 8e) increase in the order of chert, pelagic clay, turbidite, terrigenous sediment, and carbonate sediment.e chert shows the lowest ξ Tlc value (~20 mol kg -1 ), with the other sediment types having ξ Tlc values of >100 mol kg -1 .e trends are similar for the Nankai subduction zone (Supplementary Fig. 8f), where the calculated ξ Tlc values increase in the order of chert (8-30 mol kg -1 ), pelagic clay, carbonate sediment, turbidite, and terrigenous sediment.
e calculations indicate the effect of subducting sediment on talc formation in the forearc mantle.In both subduction zones, carbonate sediment and chert have low ξ Tlc values (Supplementary Fig. 8e, f).However, fluid flux might also control the formation of talc in the forearc mantle.e calculations considered only the latter effect.e carbonate sediment and chert in both subduction zones have low H 2 O contents and show minimal H 2 O loss with increasing pressure (i.e., with increasing depth; Supplementary Fig. 8c, d).erefore, fluid flux from carbonate sediment and chert would be limited, as previously suggested 21 , and subduction of these sediments might not result in significant talc formation in the forearc mantle.
In the northeastern Japan and Nankai subduction zones, the calculated H 2 O contents of the pelagic clay, terrigenous sediment, and turbidite show a decrease with increasing pressure (i.e., depth; Supplementary Fig. 8c, d), suggesting these types of sediment may be dehydrated during subduction.e calculated ξ Tlc values for these sediments are larger in northeastern Japan than at Nankai (Supplementary Fig. 8e, f), suggesting that the geotherm is the primary control on talc formation, as suggested from the results presented in Supplementary Discussion 7.Moreover, in both subduction zones, ξ Tlc increases in the order of pelagic clay, turbidite, and terrigenous sediment (Supplementary Fig. 8e, f).erefore, the generation of even a small amount of fluid from subducting pelagic sediment may be sufficient for talc formation in the forearc mantle.

Prediction of the mineral assemblage in forearc mantle of the Cascadia subduction zone
We applied our thermodynamic approach to the Cascadia subduction zone.e chemical compositions of subducting sediment are provided in Supplementary Table 1.For the modeling of subducting sediment, we used the Cascadia sediment composition of Plank 19 , although carbon contents were not provided.We used the organic and inorganic carbon contents of Cascadia sediment reported by Cli 14 in addition to the bulk compositions from Plank 19 .e Fe 3+ /∑Fe ratio was set to 0.23, based on the global average for metapelites 20 .e calculations were conducted along the slab-top geotherm of the Cascadia subduction zone.e subduction velocity (v) was set to 42,000 m Myr -1 [14] and we used the thickness of subducting sediments (h = 400 m) from ref. 18 .
Fluids in equilibrium with subducting sediment in the Cascadia subduction zone are carbon-rich, with C concentrations increasing from 1.9 mol kg -1 at 25 km depth to 11.6 mol kg -1 at 90 km (Supplementary Fig. 9a).e Si concentrations are an order of magnitude lower than the C concentrations, increasing from 0.1 mol kg -1 at 25 km depth to 4.5 mol kg -1 at 90 km.e mineral assemblage of the subducting sediment (Supplementary Fig. 9b) indicates that the amount of H 2 O in the rock decreases from 1.6 to 1.5 wt% at 31 to 34 km depth, and from 1.7 to 1.2 wt% at 41 to 73 km depth.ese changes respectively correspond to the breakdown of chlorite + epidote + biotite and amphibole + mica.e calculated fluid flux from subducting sediment is 0.5-9.1 × 10 3 kg m -2 Myr -1 from 31 to 34 km depth (Supplementary Fig. 9c).e fluid flux at depths below the island arc Moho (i.e., 35 km) increases from 0.7 × 10 3 kg m -2 Myr -1 at 43 km depth to 13.9 × 10 3 kg m -2 Myr -1 at 58 km depth, and subsequently decreases to 1.6 × 10 3 kg m -2 Myr -1 at 73 km depth.
We now consider the predicted mineral assemblage in the forearc mantle of the Cascadia subduction zone for t/L =1.e talc fraction increases from 2 to 60 vol% at 42-52 km depth (Supplementary Fig. 9d), and carbonate minerals (magnesite + dolomite) increase from 5 to 52 vol% (Supplementary Fig. 9e).e mineral assemblage at 54-65 km depth was not calculated due to numerical instability.Talc + serpentinite + carbonate (magnesite) is predicted to occur at depths of 67-69 km, and carbonate minerals are predicted to occur at 73 km depth without hydrous minerals (Supplementary Fig. 9d, e).Overall, our model predicts a talc + carbonate assemblage in the Cascadia and Nankai subduction zones (Fig. 2g, h).In the former, the proportions of talc and magnesite show similar increases with depth (Supplementary Fig. 9d, e), suggesting these minerals are formed mainly by the infiltration of carbon-rich fluid (reaction 2) rather than silicarich fluid (reaction 1).

Changes in seismic properties in the response to infiltration of metasediment-derived fluid into serpentinites
e P-and S-wave velocities for zero-porosity mineral aggregates were calculated using the program MinVel 22 , which is a customized code based on ref 23 .e P-and S-wave velocities were calculated by excluding the hematite that was observed in our thermodynamic calculations because the MinVel dataset does not include hematite.
e thermodynamic model results were used to predict seismic velocities from the mineral proportions.For example, as the mantle wedge reacts with fluid originating from subducting sediments, the mantle wedge serpentinite is gradually modified to a rock consisting mainly of talc + carbonate (magnesite; Supplementary Fig. 10a).e serpentinite has V P = 6.7-7.0 km s -1 and V S = 3.8-3.9km s -1 , whereas the talc + magnesite rock has V P = 7.0 km s -1 and V S = 4.2 km s -1 (Supplementary Fig. 10b), suggesting that the changes in mineral assemblages in response to fluid infiltration would not result in significant changes in V P and V S .Moreover, V P overlaps values for anhydrous (8.0-8.4 km s -1 ) to fully hydrous mantle (6.6 km s -1 ; Supplementary Fig. 10b) 6 .erefore, distinguishing serpentinite and talc-carbonate rock solely based on seismic observations would be difficult, as previously suggested 24 , and the low-velocity layer in the Nankai subduction zone could also be due to talc-carbonate layers at the bottom of the mantle wedge.

Supplementary
).In general, m Si values are high in the Nankai subduction zone (warm subduction zone) as compared with the northeastern Japan subduction zone (cold subduction zone).Supplementary Fig.7g-i shows a summary of the calculations of infiltration of sediment-derived fluids into the mantle rocks under various P-T conditions along the slab-top of the Nankai and northeastern Japan subduction zones.In northeastern Japan, the ξ Tlc values for various F OC values are 60-300, 45-250, and 35-189 mol kg -1 for sediments for 0.41, 0.82, and 1.62 wt% TC, respectively.In contrast, in the Nankai subduction zone, the ξ Tlc values for various F OC values are 10-182, 8-171, and 8-200 mol kg -1 for sediments for 0.41, 0.82, and 1.62 wt% TC, respectively.e ξ Tlc values are typically low for the Nankai subduction zone as compared with those for the northeastern Japan subduction zone, suggesting the geotherm is the primary control on the efficiency of talc formation.Furthermore, the ξ Tlc values become smaller with increasing TC in the sediments in northeastern Japan, whereas this trend is less clear for the Nankai subduction zone.erefore, increasing TC in the subducting sediments requires less fluid for talc formation in a cold subduction zone.

Figure 9 .
| Model predictions for the Cascadia subduction zone.a Composition of fluid in equilibrium with metasedimentary rocks along the slab-top P-T path.b Mineral phases and H 2 O and C contents in the metasedimentary rocks.c Flux of fluid derived from subducting sediments.d-e Mineral proportions (representative mineral (d) and carbonate (e)) at the base of the mantle wedge adjacent to subducting sediments.Mineral proportions in regions shallower than the island arc Moho are shown in light colors for reference.MDD = maximum depth of decoupling 1 ; SSE = slow slip event 26,27 ; ETS = episodic tremor and slip 26,27 .N.D. (not determined) indicates the mineral proportions were not calculated due to numerical instability.In c-e, the blue and white backgrounds indicate the depths at which dehydration was predicted and not predicted, respectively.Srp = serpentine; Tlc = talc; Chl = chlorite; Mgs = magnesite; Dol = dolomite.

Table 4 . | Summary of calculation settings for parameter studies.
F OC = OC/(OC + IC) (in mass units).38 Supplementary